In certain wireless communication systems, a wireless communication device (WCD) can transmit data at varying data rates. For example, an WCD's uplink data rate could depend on the frequency bandwidth, modulation, and coding schemes used in the uplink transmissions. These factors can also affect the power needed for the uplink transmissions. In the case that orthogonal frequency division multiple access (OFDMA) is used for the uplink, for example, in Long Term Evolution (LTE) air interfaces, the transmitted power for a given modulation and coding scheme can be a function of the frequency bandwidth that is used. Thus, if the modulation and coding scheme remains fixed, a WCD can transmit at a higher uplink data rate by using a greater frequency bandwidth and more transmit power.
In the LTE approach, the network typically allocates uplink resources to a WCD in the form of one or more resource blocks. Each resource block has a duration of 0.5 milliseconds (a period of time that may be referred to as a slot) and has a total of 12 sub-carriers, with each sub-carrier occupying 15 kHz of frequency bandwidth. A WCD could be allocated more than one resource block for a given slot, and the WCD's transmit power requirement could scale with the number of resource blocks. For example, the transmit power requirement for a given WCD transmitting into an uplink channel, may be calculated as follows:P=P0+αL+Δ+10 log10M  (1)where P is the transmit power requirement (in dB), P0 is a base level, α is a path-loss compensation factor, L is a path-loss estimate, Δ is a dynamic offset, and M is the number of resource blocks allocated to the WCD.
While a WCD's transmit power requirement may increase with the number of resource blocks allocated to it, the WCD may also be associated with a maximum transmit power. The maximum transmit power could, for example, be based on limitations of the WCD's hardware, software, or firmware and/or based on a limitation set by the network. In the LTE approach, a WCD may calculate a “power headroom” as a difference between the WCD's transmit power requirement for its resource block allocation and the WCD's maximum transmit power, and the WCD may report the power headroom that it calculates to the network. The power headroom that a WCD calculates could be positive, for example, when the WCD's transmit power requirement is less than its maximum transmit power. However, the power headroom could also be negative, for example, when the WCD has been given a resource block allocation with a transmit power requirement that exceeds its maximum transmit power.
The network may use the power headrooms reported by the WCDs operating in a wireless coverage area to adjust the uplink resources that are allocated to those WCDs. For example, a WCD that reports a positive power headroom could be allocated a greater number of resource blocks, whereas a WCD that reports a negative power headroom could be allocated a fewer number of resource blocks.